Where should I travel solo?

I recently started a new job which leaves me with more flexibility and days off. I am thinking about making use of this to plan a solo trip somewhere, something I haven’t done since my eldest son was born almost 12 years ago.

I live in MI and find that Detroit airport has the best mix of affordable flights and location convenience for me, so I plan to fly out of DTW. I have school aged children and don’t want want to leave them too long so this will likely be a short trip ~ 5 days. I am considering Mexico City; I haven’t been there in 18 years and it would be fun to see what I remember. We also talked about traveling there with the kids and this would help me scope it out first.

Does anyone have any other good ideas?
Criteria:

<8 hrs travel time

<$500 plane ticket

Lots to see and do in a centralized area

Good public trans or pedestrian/bike friendly

Affordable lodging (condo rental…hostel)

I’d love to hear your suggestions. Leave them in the comments. Thank you!